In wireless communication systems, antenna diversity plays an important role in increasing the system link robustness. OFDM is used as a modulation technique for transmitting digital data using radio frequency signals (RF). In OFDM, a radio signal is divided into multiple sub-signals that are transmitted simultaneously at different frequencies to a receiver. Each sub-signal travels within its own unique frequency range (sub-channel), which is modulated by the data. OFDM distributes the data over multiple channels, spaced apart at different frequencies.
OFDM modulation is typically performed using a transform such as Fast Fourier Transform (FFT) process wherein bits of data are encoded by an encoder in the frequency-domain onto sub-channels. As such, in the transmitter, an Inverse FFT (IFFT) is performed on the set of frequency channels to generate a time-domain OFDM symbol for transmission over a communication channel. The IFFT process converts the frequency-domain phase and amplitude data for each sub-channel into a block of time-domain samples which are converted to an analogue modulating signal for an RF modulator. In the receiver, the OFDM signals are processed by performing an FFT process on each symbol to convert the time-domain data into frequency-domain data, and the data is then decoded by a decoder by examining the phase and amplitude of the sub-channels. Therefore, at the receiver the reverse process of the transmitter is implemented. Further, transmit antenna diversity schemes are used to improve the OFDM system reliability. Such transmit diversity schemes in OFDM systems are encoded in the frequency-domain as described.
With the increase in transmission rates, higher operational speeds in the encoder and decoder are resulting in difficulties in implementing such channel encoders/decoders. The transmitter architecture with only one encoder is adopted in current IEEE 802.11n (high throughput WLAN) proposals. FIG. 1 shows one of the examples for such designs implemented in a transmitter 100. The transmitter 100 includes an FEC encoder 102, a puncture unit 104, a spatial stream parser 106, and multiple stream processing paths. Each stream processing path performs the functions of: frequency interleaving 108, QAM mapping 110, antenna mapping 112, IFFT operation 114, inserting guard interval (GI) 116, analog RF modulation 118 and antenna 120. In the transmitter 100, a data stream is first encoded using the FEC encoder before it is split into multiple spatial streams by the spatial stream parser. Multiple interleaving functions with different frequency rotation values are applied after the spatial parsing. As there is only one encoder in the coding chain, there is only one decoder in the decoding chain at a receiver (not shown).
The interleaver design in the high throughput wireless local area network (WLAN) systems is an important issue for MIMO-OFDM transmission. In the current approaches in IEEE 802.11n standards (S. A. Mujtaba, “TGn Sync Proposal Technical Specification,” a contribution to IEEE 802.11, 11-04/0889r4, March 2005, and C. Kose and B. Edwards, “WWiSE Proposal: High throughput extension to the 802.11 Standard,” a contribution to IEEE 802.11, 11-05-0149r2, March 2005, incorporated herein by reference), the transmitter architecture with single channel encoder is considered regardless of the number of the transmit antennas. When the data rate or the number of the data streams increases, the encoder/decoder must operate with very high speed, causing circuit design and timing implementation difficulties.